Electroluminescent (EL) devices based on organic thin layers have recently attracted much attention because of their potential uses in large-area flat-panel displays and light-emitting diodes (LED). Organic LEDs have been made with both low molecular-weight organic materials and with polymers. The performance of these devices is significantly influenced by the charge balance between electrons and holes from opposite electrodes. The charge can be balanced by using a bilayer structure including a hole transporting layer and an electron transporting layer. One or both of these layers can be luminescent.
An important quality of organic EL materials is their durability, i.e., thermal and morphological stability. Thus, it is desirable that organic EL materials are not only light-emitting and hole transporting, but also robust.
The hexasubstituted benzene compounds of the invention are useful as hole transporting, green-light-emitting molecules with high glass transition temperatures. These compounds have a number of qualities that make them useful in electroluminescence devices.
In one aspect, the invention features a method of preparing a compound of formula I: 
Each of R1-R6 is, independently, 
Y is O, S, NH, or C(R7)=C(R8). Each of R7-R11 is, independently, H, substituted or unsubstituted C1-6 alkyl substituted or unsubstituted C2-6 alkenyl, substituted or unsubstituted C2-6 alkynyl, substituted or unsubstituted C6-20 aryl, or substituted or unsubstituted C4-20 heteroaryl. Alternatively, each of R7-R11 is OH, C1-6 alkoxy, or N(R12)(R13). For N(R12)(R13), each of R12 and R13 is, independently, H, substituted or unsubstituted C1-6 alkyl, substituted or unsubstituted C2-6 alkenyl, substituted or unsubstituted C2-6 alkynyl, or substituted or unsubstituted C6-20 aryl. Each of R7--R11 can also be NO2, CN, or CO2R14, in which R14 is H or C1-6 alkyl.
The method includes contacting a compound of formula II with a compound of formula III to form a compound of formula I. The compound of formula II is shown below: 
In this compound, each of X1-X6 is, independently, Br or I. The compound of formula III is shown below: 
In this compound, Y and R7-R11 are as defined above and Z is ZnCl or Sn(R15)(R16)(R17), in which each of R15-R17 is, independently, C1-6 alkyl.
In some preferred embodiments, Y is O, S, or NH; each of R10 and R11 is H; and/or R9 is N(R12)(R13). Preferably, each of R12 and R13 is, independently, substituted or unsubstituted aryl; for example, each of R12 and R13 can be, independently, phenyl, tolyl, naphthyl, or pyrenyl. In other preferred embodiments, R9 is carbazolyl. In other preferred embodiments, Y is C(R7)xe2x95x90C(R8). In still other preferred embodiments, each of R15-R17 is methyl or butyl.
In another aspect, the invention features a method of forming a hexaarylbenzene; the method includes contacting a hexahalobenzene with a stannane. In preferred embodiments, the hexaarylbenzene is a hexakis-(heteroaryl)benzene. For example, the hexaaryylbenzene can be a hexakis-(thienyl)benzene. e.g., a hexakis-(carbazolylthienyl)benzene, or a hexakis-(aminothienyl)benzene, e.g., a hexakis-[(diarylamino)thienyl]benzene.
In another aspect, the invention features a compound of formula I 
Each of R1-R6 is, independently, 
in which Y is O, S, or NH. Each of R9-R11 is, independently, H, substituted or unsubstituted C1-6 alkyl, substituted or unsubstituted C2-6 alkenyl, substituted or unsubstituted C2-6 alkynyl, substituted or unsubstituted C6-20 aryl, or substituted or unsubstituted C4-20 heteroaryl. Alternatively, each of R9-R11 is OH, C1-6 alkoxy, or N(R12)(R13). For N(R12)(R13), each of R12 and R13 is, independently, H, substituted or unsubstituted C1 6 alkyl, substituted or unsubstituted C2-6 alkenyl, substituted or unsubstituted C2-6 alkynyl, or substituted or unsubstituted C6-20 aryl. Each of R9-R11 can also be NO2, CN, or CO2R 14, in which R14 or C1-6 alkyl.
In some preferred embodiments, each of R10 and R11 is H. In other preferred embodiments, R9 is N(R12)(R13), and each of R12 and R13 is, independently, substituted or unsubstituted aryl, e.g., phenyl, tolyl, naphthyl, or pyrenyl. In still other preferred embodiments, R9 is carbazolyl.
Preferably, Y is S. For example, each of R1-R6 can have one of the following formulae: 
In another aspect, the invention features an electroluminescence device made with one of more of the compounds described above. The device includes a substrate, which may be coated. The device also includes a hole transporting layer, an emitting layer, and an electron transporting layer. The compounds described above may be included in the hole transporting layer and/or the emitting layer.
The term xe2x80x9csaturatedxe2x80x9d used herein refers to a compound or portion of a compound having each atom either hydrogenated or substituted such that the valency of each atom is filled.
The term xe2x80x9cunsaturatedxe2x80x9d used herein refers to a compound or portion of a compound where the valency of each atom may not be filled with hydrogen or other substituents. For example, adjacent carbon atoms can be doubly bound to each other.
The term xe2x80x9csubstitutedxe2x80x9d used herein refers to moieties having one, two, three or more substituents, which may be the same or different, each replacing a hydrogen atom. Examples of substituents include but are not limited to alkyl, hydroxyl, protected hydroxyl, amino, protected amino, carboxy, protected carboxy, cyano, alkoxy, and nitro.
The term xe2x80x9cunsubstitutedxe2x80x9d used herein refers to a moiety having each atom hydrogenated such that the valency of each atom is filled.
The term xe2x80x9carylxe2x80x9d used herein refers to a moiety having a hydrocarbon ring system (e.g., a fused ring system) having at least one aromatic ring. Examples of aryl moieties include, but are not limited to, phenyl, naphthyl, and pyrenyl.
The term xe2x80x9cheteroarylxe2x80x9d used herein refers to a moiety having a ring system (e.g., a fused ring system) with at least one aromatic ring and at least one heteroatom, including, but not limited to, O, N, and S. Examples of heteroaryl moieties include, but are not limited to, pyridinyl, carbazolyl, and indolyl.
Protected forms of the compounds described herein are included within the scope of the invention. In general, the species of protecting group is not critical, provided that it is stable to the conditions of any subsequent reaction(s) on other positions of the compound and can be removed at the appropriate point without adversely affecting the remainder of the molecule. In addition, one protecting group may be substituted for another after substantive synthetic transformations are complete. Examples and conditions for the attachment and removal of various protecting groups are found in T. W. Greene, Protective Groups in Organic Chemistry, (1st ed., 1981, 2nd ed., 1991).
In addition, salts of the compounds described herein are within the scope of the invention. For example, a salt can be formed between a positively charged amino substituent and a negatively charged counterion.
The details of several embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.
The invention features methods for preparing hexasubstituted benzene compounds, the compounds themselves, and EL devices made using these compounds. In particular, the invention features methods for the six-fold substitution of a hexahalobenzene. For example, a hexakis(thienyl)benzene can be synthesized by the palladium-catalyzed thienylation of hexabromobenzene. These starburst-shaped compounds can help improve the physical properties of the organic LEDs into which they are incorporated.
A method for synthesizing a hexaarylbenzene is as follows: If the aryl group is to be substituted, a substituted aryl or heteroaryl compound is first prepared. The substituted aryl or heteroaryl compound may be prepared by coupling a halogenated aryl or heteroaryl compound with a substituent in the presence of a catalyst, e.g., CuI. Examples of substituents include alkyl, aryl, amino, alkoxy, hydroxy, carboxy, nitro, and cyano substituents. The resulting compound is then converted to a metallated compound, e.g., an aryl zinc chloride, a heteroaryl zinc chloride, an aryl stannane, or a heteroaryl stannane. A hexahalobenzene is then contacted with the metallated compound, or a mixture of such compounds, in the presence of a catalyst. The result is a hexasubstituted benzene.
Shown below in the scheme is a synthetic procedure for making exemplary compounds of the invention, indicated as compounds 4a-4d. 
As shown in the scheme, Ullmann coupling of iodothiophene with diarylamines 1a-1d affords diarylthienylamines 2a-2d. Amines 2a-2d are then converted to thienylstannanes 3a-3d. (P. V. Bedworth, Y. Cai, A. Jen, S. R. Marder, J. Org. Chem. 1996, 61, 2242) Sixfold thienylation of hexabromobenzene with thienylstannanes 3a-3d, using Stille""s cross-coupling reaction (Stille, J. K., Angew. Chem. Int. Ed. 1986, 25, 508) yields air-stable starburst compounds 4a-4d.
Hexaarylbenzene derivatives are useful as organic EL materials for a variety of reasons. The expected twisting of the aryl units to the central benzene may hinder close packing of the molecules in the solid state, and facilitate formation of stable amorphous morphology. In addition, the high local concentration of functional groups, which may act as hole transporting units, may be beneficial to the physical performance of the materials. For example, the concentration may promote current flux.
The electronic properties of the aryl groups are important as well. For example, the xcfx80-excessive thiophene ring may lower the oxidation potential of the compound to which it is attached.
The compounds of the invention can be used to make electroluminescence devices. A diagrammatic representation of such a device is shown below. 
Electroluminescence devices generally include multiple layers. A typical device includes a substrate, e.g., glass, which may be coated with an oxide, e.g., indium-tin-oxide (ITO). The device also includes a hole transporting layer, an electron transporting layer, and an emitting layer. The hole transporting layer and the emitting layer may be combined into a single layer, or the emitting layer and the electron transporting layer may be combined into a single layer. The device may also include a cathode.
Devices can be prepared by vacuum deposition of compounds 4a-4d (as hole transporting layer), followed by Alq3 as emitting layer and electron-transporting layer (Alq3=tris(8-quinolinolato)aluminum (III), C. W. Tang, S. A. VanSlyke, Appl. Phys. Lett. 1987, 51. 913; J. Kido, Y. Lizumi, Chem. Lett. 1997, 963) onto an indium-tin-oxide (ITO) coated glass substrate. An alloy of magnesium and silver (ca. 8:1, 500 xc3x85), which serves as the cathode, can be deposited onto the organic layer by simultaneously evaporating from two different sources. The cathode is capped with 1000 xc3x85 of silver. The current-voltage (I-V) curve can be measured on a Keithley 2000 Source Meter in an ambient environment. Light intensity (L) is measured with a Newport 1835 Optical Meter.